The present invention is directed to an improved plating method. More specifically, the present invention is directed to an improved plating method where adjustments of the plating cycle improve throwing power and reduce nodule formation.
Generally, plating substrates with metals includes passing a current between two electrodes in an electrolyte where one of the electrodes is the substrate to be plated. Electrolytes for depositing a metal on a substrate typically include one or more metal ions, a soluble salt in a sufficient amount to impart conductivity to the electrolyte, and additives to improve plating uniformity and quality of the metal deposit. Such additives may include brighteners, levelers, suppressors, antioxidants, and surfactants.
In many conventional plating processes electrodes (cathode and anode) of an external circuit are immersed into the electrolyte and a DC (direct current) is applied across the electrodes. This causes an electrochemical reaction or reduction resulting in deposition of a metal or metal alloy onto a cathode from metal ions in the electrolyte. Current density profile and primary distribution across the cathode varies according to the geometric path or distance between the anode and the cathode leading to deposit thickness variation according to shape and location of the cathode relative to the anode. This effect is most apparent when high applied average current densities are used. Therefore, in order to obtain the best uniformity of metal distribution, low applied average current densities are used.
Alternatively, by using PPR (pulse periodic reverse electroplating) current instead of DC current, uniform metal deposits may be produced at higher current densities. This technique is especially useful for electrolytic copper plating on high aspect printed wiring boards, which are relatively thick boards with small through-hole diameters. Such substrates present plating problems because of their surface geometry, which affects current distribution, and results in measurable differences in current density between the surfaces of the board and the through-holes. The current density difference causes uneven metal deposition with thicker coatings produced on surfaces with higher current densities. Generally, board edges and isolated surface circuitry experience higher current density and result in thicker deposits compared to the center surfaces of the board or the inner surface of the through-holes (sometimes referred to as dog-boning). Additional thickness in these areas may present problems in subsequent processing and assembly operations. A non-uniform surface profile may lead to increased soldermask being required to meet minimum thickness requirements for suitable coverage. A lack of circuit planarity and excess thickness at through-hole entries may interfere with proper location of components during assembly, while methods used to reduce this excess thickness may lead to protracted processing times and a loss of production.
PPR current may produce metal deposits with an even thickness on both the board surface and in the through-holes. A PPR current is created by alternating current modulation between forward and reverse cycles. This is accomplished by inverting the current from cathodic to anodic mode, which disrupts the otherwise constant direct current polarization effects. The degree of disruption occurs according to the primary current distribution with more in the high current density areas than in the low current density areas, thus providing a normalization of deposition rates across complex geometries at higher applied average current densities. Moreover, by maintaining thickness uniformity at higher applied average current densities, the overall metal deposition rate is increased and processing times reduced yielding higher production output.
Although the use of PPR may result in uniform deposit thickness at high current densities, the surface appearance of the resulting deposit may range from a matte to a semi-bright finish relative to the through-hole wall, thus producing a non-uniform deposit appearance between high (surface) and low (through-hole) current densities. On the other hand, if DC current is applied, uniformly bright deposits are typically produced throughout the current density range, but low current densities are used in order to preserve metal deposit thickness uniformity. Accordingly, neither method provides optimum thickness distribution with uniform metal deposit appearance at high current densities.
Metals that may be plated include, for example, copper, copper alloys, nickel, tin, lead, gold, silver, platinum, palladium, cobalt, chromium, and zinc. Electrolytes for metal plating are used for many industrial applications. For example, they may be used in the automotive industry as base layers for subsequently applied decorative and corrosion protective coatings. They also may be used in the electronics industry, such as in the fabrication of printed circuit or wiring boards, and for semiconductor devices. For circuit fabrication in a printed circuit board, a metal such as copper is plated over selected portions of the surface of a printed circuit board and onto the walls of through-holes passing between the surfaces of the circuit board base material. The walls of the through-holes are metallized to provide conductivity between circuit layers on each surface of the board.
U.S. Pat. No. 6,402,924 discloses a method for depositing a metal onto a substrate which has apertures or uneven surfaces. The method improves the surface appearance including brightness, grain structure and through-hole leveling of the deposit while maintaining throwing power at high current densities. Optimum throwing power is achieved when the plating current density at the center of the through-hole is the same as that flowing at the substrate surface. Such a current density is desired, but rarely achieved, to provide for uniform metal layers at the surface of the substrate and in the through-holes. Circuit defects may occur when the current density at the surface of the substrate is different from that of the through-holes.
The method of depositing a metal on the substrate disclosed in the '924 patent involves applying a pulsed periodic reverse current across the electrodes of a plating cell utilizing a peak reverse current density and a peak forward current density, and varying the ratio of the peak reverse current density to the peak forward current density in periodic cycles to provide a metal deposit of uniform appearance, fine grain structure and uniform metal thickness onto the substrate. One way to vary this ratio is by holding the peak forward current constant while varying the peak reverse current density.
The metal which is deposited onto the substrate depends on the application. For example copper is generally used as an undercoat for protection and conductivity while gold may be used as a topcoat for decoration, protection and function such as for electrical contacts. Copper and gold alloys also may be plated with this method. Other metals which may be deposited by the method include tin, lead, palladium, nickel, silver, zinc, and their alloys. The method is typically used to deposit copper onto printed circuit boards with high aspect ratios, where aspect ratio is board thickness divided by through-hole diameter.
While the method disclosed in U.S. Pat. No. 6,402,924 addresses many of the problems discussed above in metal plating, the printed circuit board industry continuously seeks greater circuit densification, thus demanding further improvements in metal plating. To increase density, the industry has resorted to multi-layer circuits with through-holes or interconnections passing through multiple layers. Multi-layer circuit fabrication results in an overall increase in the thickness of the board and a concomitant increase in the length of an interconnection passing through the board. This means that increased circuit densification results in increased aspect ratios and through-hole length and an increase in the severity of, for example, the dog boning problem. For high density boards, aspect ratios may exceed ten to one.
Another problem encountered in metal plating is the formation of nodules, also called dendrites, on the metal deposit. Nodules are believed to be crystals of the metal being plated and grow out of the plated surface. Although the cause of nodules has been the subject of some debate, nodules typically appear when there are incomplete suppressor layers on the substrate. Suppressors generally provide a large change in the kinetic overpotential of the deposition reaction. This tends to give a more uniform current distribution over the surface of the substrate and allows the metal deposition to proceed with a global leveling. Suppressors adsorb onto many metals such as copper and are not typically consumed during the metal deposition reaction. Suppressors may be distinguished from levelers, which also increase surface overpotential but are consumed or altered during metal deposition. Generally, suppressors are high molecular weight oxygen containing polymers such as polyethylene oxide, polypropylene oxide, co-polymers (random and block) of the monomers of the preceding polymers, and other surfactant molecules.
Nodules may range in diameter of from less than 1 micron to as large as several millimeters. Nodules are undesirable for a variety of electrical, mechanical, and cosmetic reasons. For example, nodules are readily detached and carried by cooling air flows into electronic assemblies, both within and external to electronic article housings, where they may cause short-circuit failure. Accordingly, the nodules have to be removed before the plated substrates are assembled into electronic articles. Conventional methods of removing the nodules involve laser inspection of each metal plated substrate followed by manual removal of the nodules by workers using microscopes. Such conventional methods leave room for worker error and are inefficient.
Accordingly, there is a need for an improved method of depositing metals and metal alloys on substrates which increase throwing power and reduce nodule formation.